A disc refiner generally comprises two opposed refining discs, which are provided with exchangeable refining elements which constitute the refining surfaces of the refiner. In disc refiners in which wood chips are to be refined into paper-making pulp, the refining is carried out between the two refining discs, which are thus kept at a definite distance from each other. Depending on the type of refiner being utilized, one or both of the refining discs are mounted on a rotary axle. These axles are driven by motors which are intended to rapidly rotate the refining discs, and the distance between the refining discs (gap) is adjusted by means of hydraulics, and is measured by means of specialized measuring systems. Due to faulty functioning during operation, the refining surfaces may contact each other. If this occurs, breakdown may result, or in any event, the refining surfaces will be subjected to considerable wear, which can significantly reduce the operating time for these refiners. It is, therefore, very important to accurately control the gap between the refining discs.
In order to accurately measure the distance between the refining surfaces, measuring systems have been employed which require preliminary adjustment of the zero point; for example, immediately after the refining elements have been exchanged or replaced. In order to so determine the zero point of the measuring system, it is important that the contact position be determined. It has been known that the contact position can be detected by utilizing sound measuring apparatus. This method requires that a transmitter be mounted on one of the two refining surfaces. When the refining surfaces then contact each other, vibrations are propagated through the refining disc to the transmitter, which can constitute a microphone, impact pulsometer or vibrometer.
One disadvantage of this method is that the transmitter also measures other sources of interference, such as the axle bearings. It is therefore difficult to detect a slight contact, and it is necessary for the signal to "drown" out other sources of interference. This technique is also incapable of measuring or determining the phase position of the contact point, i.e., the point or location where the refining surfaces first come in contact with each other. Another disadvantage of these techniques is that they presume that one of the two refining surfaces is stationary. Therefore, there are no present day methods for detecting the contact point in the case of a pair of rotating refining surfaces.